1. Field of the Invention
A process for reducing trace levels of metallic impurities in cyclized polyisoprene resin by washing an impure organic solvent/cyclized polyisoprene resin solution first at least once with a heated aqueous acidic solution and subsequently washing at least once with heated water.
2. Brief Description of Prior Art
The microelectronics industry requires that all materials used in a manufacturing environment be extremely clean with respect to trace metals. These metals, when present, are very harmful to the performance of microelectronic devices.
Cyclized polyisoprenes are used as a major ingredient in negative-making photoresists. These are conventionally made by cyclizing polyisoprene in an organic solvent (e.g., xylene) using a Lewis acid catalyst in a stainless steel reactor. Generally, a dilute polyisoprene resin solution [e.g., 10% by weight solids in the organic solvent (e.g., xylene)] is charged into the reactor. The Lewis acid catalyst dissolved that organic solvent (e.g., BF.sub.3 etherate in xylene) is then charged to the reactor. The resulting reaction cyclizes the polymer as well as decreases the viscosity of the solution. The viscosity of the reaction solution is monitored until a desired value is reached; at this time, a quenching agent (e.g., methanol or water) is added to the reaction solution to slow down or stop the cyclization reaction. After the quench, the solution is warmed until the quenching agent can be removed by vacuum stripping. This vacuum proceeds until the quenching agent is removed from the reaction solution. Optionally, a portion of the remaining organic solvent may be stripped further from the cyclized polyisoprene solution to produce a final solution having the desired solids content.
The above process has certain deficiencies associated with it. The Lewis acid catalyst has the tendency to generate protons that may corrode the reactor surfaces as well as the other metallic materials involved in the process (e.g., column condensers). This undesirable corrosion may import high concentrations of trace metals into the final cyclized polyisoprene resin solution. In addition, the presence of any remaining Lewis acid catalyst trace impurities in the final product can cause undesirable further cyclization reactions which may cause a drift in the product's properties and the final resist's properties over time.
Accordingly, there is a need for removing trace levels of such metallic impurities and any additional Lewis acid catalyst impurities from a cyclized polyisoprene resin solution without significantly altering the fundamental properties of the resin. The present invention answers that need.